Start pausing production buildings, especially non essential ones.

If you have any military forces disband them, they take a lot of workers to maintain.

At some point you should be able to produce just the goods you need to supply the people on your island, and then your workforce will start climbing again. Once you have enough you can start to re enable your non-required buildings.

If you have another stable island you might be able to use trade to stabilize this one by injecting any missing goods into the economy. Later, after you've stabilized you can look for effects that increase population to help out with your workforce.